SENSIBILITY CF VOLVOX TO SPECTRAL LIGHTS 365 



vision) coincides with the curve of visibiHty for totally color- 

 blind persons (isolated rod vision), this latter being the same as 

 the achromatic scotopic curve, we conclude that the rods are 

 always stimulated in the same relative degree. High and low 

 intensities produce the same relative effects as to wave-lengths. 

 Applied to lower organisms, we conclude that there is nothing 

 in the sensibility to light of different wave-lengths which would 

 lead us to assume anything comparable to the photosensitive 

 substance in the cones, and that no matter with what intensity 

 of the spectrum we work, we will always obtain the same rela- 

 tive sensibility wave-length curve. 



If a substance is sensitive to light of a particular wave-length, 

 it must absorb this light and show an absorption band in the 

 region in question, the absorption spectrum of a chemical system 

 being intimately connected with its photochemical behavior. 

 Light waves are absorbed in ponderable media by particles 

 capable o'' a free period of vibration. Vibrations not synchro- 

 nized to these produce only forced vibrations of the particles and 

 would hence be only slightly absorbed. 



In a heterogeneous mixture, if the substances do not interact 

 to form a new combination, the light absorption of a mixed solu- 

 tion will be equal to the sums of the absorptions which the com- 

 ponents would exert separately. That is, when chemical inter- 

 actions are excluded, the behavior of summed absorptions are 

 purely additive. 



The photochemical substances in organisms are probably het- 

 erogeneous, comprising a number of different substances, so that 

 a number of different groups of wave-lengths are absorbed, pro- 

 ducing, by resonant vibration of different rates, chemical re- 

 actions, resulting in photochemical products. That is, in the 

 formation of particular photochemical products, vibrations (mo- 

 lecular, atomic, or electronic) of a certain rate are excited by 

 resonance, with excitation as the result of the energy set free. 



For recent applications of this chemical and physical conception 

 involving absorption, vibration and resonance in theories of color 

 vision, see the papers by Houstoun, '16; Guild, '18, and Troland, 

 '16 and '17. 



